Disclosed is a fibrous solid substrate in pellet form suitable for use in agriculture. The fibrous solid substrate in pellet form is obtained from spent biomass material following an anaerobic fermentation and biogas production. Further disclosed is a method for cultivating fungal cells and/or spores using said fibrous solid substrate in pellet form. Also disclosed are methods for upgrading spent biomass material from a biogas production unit to fertilizer products, fibers and substrate for biogas.
